1. Field of the Invention
This invention relates to an improvement in carburization process, in particular, plasma carburization process, for forming a carburized layer on the surface of the workpiece with the edge provided (for example, gears).
2. Description of Related Art
Plasma carburization is a process for carburizing the workpiece surface by placing the workpiece in a vacuum furnace, heating it to 850.degree.-1100.degree. C., introducing hydrocarbon gas such as C.sub.3 H.sub.8 into the furnace as reaction gas, and generating glow discharge to make the workpiece a cathode (for example, Japanese Tokkaihei No. 2-145759) and higher carburization efficiency can be obtained as compared to gas carburization, and has, in general, an advantage that uniform concentration distribution is achieved throughout the whole surface even at a high carbon concentration. After plasma carburization, diffusion treatment is carried out as required to diffuse taken-in carbon inside the workpiece surface and to form a carburized layer with a specified thickness.
Carbon taken into the workpiece surface by plasma carburization gradually diffuses inside during carburization and diffusion followed and the carburized layer of a specified thickness is formed, but when this kind of plasma carburization is applied to the workpiece with an edge portion such as gears, in the carburized layer at the said edges the carbon concentration becomes relatively higher than that at the flat portion. This is attributed to the difference in shapes between the edge portion and the flat portion, and the edge portion A (see FIG. 8) has a small diffusible area inside for the size of its surface area as compared to the flat section B and carbon taken in is difficult to diffuse inside and the carbon concentration on the surface is difficult to lower.
Nonuniformity of surface carbon concentration at the edge portion A and the flat portion B is conspicuous in plasma carburization and it does not create any significant problem in gas carburization. That is, because gas carburization is carburization under the equilibrium state, the carbon concentration on the workpiece surface does not rise beyond the concentration to be equilibrated to the carbon potential in atmosphere and the carburizing speed decreases as the carbon concentration on the workpiece surface increases, and carburization takes place with the surface carbon concentration held nearly constant at the edge portion A and flat portion B. However, because plasma carburization is carburization under the non-equilibrium state and the speed to take in carbon is not so different from that at the flat portion B even if the surface carbon concentration at the edge portion A increases during carburization, the difference on surface carbon concentration after carburization tends to increase. The nonuniformity of carbon concentration is not cleared even if diffusion treatment is carried out.
Consequently, upon completion of diffusion treatment, even if the portion with the carbon concentration exceeding the eutectoid point disappears at the carburized layer of the flat portion B, there is a case in which the portion exceeding the eutectoid point remains at the carburized layer of the edge portion A because the carbon concentration is difficult to lower, and in such event, carbides in the mesh form are formed along the grain boundary at the edge portion A of the workpiece after cooling. The carbides in the mesh form formed along the grain boundary are brittle and workpiece as a crack initiation point (furthermore, the edge portion A is a portion to which stress is concentrated), and once they are formed, they are difficult to disappear even by heat treatment carried out thereafter.